1. Field of the Invention
The present invention is broadly concerned with modified synthetic polymer compositions for use in forming high strength, biodegradable plastics and methods of forming such compositions. More particularly, the inventive compositions comprise polylactic acid joined or copolymerized with starch via a linkage or compatabilizing group which comprises a diisocyanate moiety. The invention allows lower quantities of polylactic acid to be utilized while maintaining the biodegradability of polylactic acid-derived plastics and improving their mechanical properties.
2. Description of the Prior Art
Plastics (synthetic resins) are widely used and an important material in current commercial products. As more plastics having varying mechanical properties are developed, industries are finding uses for plastics which only a few years ago would have been unexpected. For example, many automobiles which were previously formed entirely of metal now include plastic components such as plastic body panels. Furthermore, these plastic components are so well-designed that it is generally difficult to visually differentiate between the plastic and steel components on an automobile. In addition to automobile components, plastics find uses in innumerable products including children's toys, kitchen dishes and appliances, packaging materials, and medical products.
While plastics have generally been an inexpensive and efficient material for manufacturing products, they are derived in large part from petroleum resources which are finite and increasing in cost. Thus, it is important to develop new methods and materials for forming plastics as an alternative to the current methods.
Additionally, the environmental impact of discarded plastic objects is of growing global concern due to the fact that disposal methods for such wastes are quite limited. Incineration of the plastic wastes generates toxic air pollution. At the same time, satisfactory landfill sites are limited, and most durable plastics do not biodegrade. There is, thus, a need for durable and biodegradable plastic materials, particularly for short-term use items such as packaging materials and disposable utensils.
Fully biodegradable, synthetic polymers have been commercially available for several years. Such polymers include polylactic acid (PLA), polycaprolactone (PCL), and polyhydroxybutyratevalerate (PHBV). Among these polymers, PLA has been extensively studied in medical implants, sutures, and drug delivery systems. Unlike other available biodegradable synthetic polymers, PLA exhibits promising mechanical properties, thus making it appealing for use as a disposable and biodegradable plastic substitute. However, PLA is costly compared to conventional petroleum-based plastics, and its degradation rate is slow compared to the waste accumulation rate. Finally, another disadvantage of PLA is that its modulus of elasticity decreases by about 85% at temperatures above its glass transition temperatures (60.degree. C.), where it becomes very soft, and thus it has only limited applications.
Starch is a renewable and degradable carbohydrate biopolymer that can be purified from various sources by environmentally sound processes. By itself however, starch has severe limitations in plastic applications due to its water solubility. That is, articles molded from starch will swell and deform upon exposure to moisture. To decrease interaction with water, starch is often blended with hydrophobic polymers which reduce the use of petroleum polymers while simultaneously increasing the biodegradability of the product.
Starch has been blended with synthetic biodegradable polymers such as PCL, PHBV, and poly(hydroxybutyrate) (PHB). However, starch and PLA have not previously been successfully blended because they are immiscible polymers. There is a need for a compatibilizer which can enhance the compatibility of starch and PLA to yield a high-strength biodegradable plastic.